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A while back I discovered a fantastic article about time travel on Slate.com. The author, Dave Goldberg, both a physicist and self-proclaimed science-fiction geek, wrote the post back in 2009 in anticipation of the film adaptation of THE TIME TRAVELER’S WIFE by Audrey Niffenegger. In the article, Goldberg takes a broad look at time travel in contemporary books and films, from BACK TO THE FUTURE to LOST to THE TERMINATOR. Ultimately, Goldberg takes the position that, at least to a physicist, THE TIME TRAVELER’S WIFE gets the science the closest to what might be considered scientifically sound.

If you’re writing a time-travel story, I highly recommend a full read of Goldberg’s article. It can be found by clicking this link. Below, for a more general overview, I’ve shared the four rules Goldberg says are necessary for scientifically sound, fictional time travel.

1) This is the only universe you’ve got.

Most of us have heard of the major scientific breakthrough of the last century known as quantum physics. One researcher in quantum physics, Hugh Everett, took the theories of quantum mechanics and imagined that perhaps the universe was constantly multiplying. He proposed the theory that, as electrons and other particles moved, the universe made almost perfect copies of itself. Since electrons and the like move constantly, these universes would be constantly multiplying, with no limit to the universes created.

This “many universes” theory lends itself well to the type of time travel depicted in BACK TO THE FUTURE, where characters travel between times and different realities are possible. A change in behavior in the past can lead to a different future. But according to Goldberg, this type of theory is at odds with Einstein’s Theory of Relativity, the very theory that suggests that time travel may be possible in the first place. Goldberg asserts that, to make your time travel fiction scientifically sound, you need to stay within the one universe you’ve been given.

2) You can’t visit any time before your time machine was built.

In Einstein’s universe, time and space are closely related to each other. Therefore, time travel could be imagined as a trip through a tunnel in space – a tunnel that has a way in and a way out – an entrance/exit at each end. The rule that follows, then, is that both ends of the tunnel need to be in a time when time travel exists.

If you think about it, the application of this rule would help explain why, if time travel is indeed possible and will one day be perfected, we don’t receive visitors who are time traveling back to us from the future. Since time travel hasn’t been “invented” yet, time travelers from the future cannot come back to our time. (I suppose it follows that, once it is invented, we can expect to meet people from the future as they come back to take a tour of our time.)

3) You can’t kill your own grandfather.

This rule concerns the consistency of history. It looks at something known as the “grandfather paradox,” which goes something like this:

Imagine you possess a time machine. You go back in time and decide to kill your own grandfather. Now what? Well, if you kill your grandfather, you will never be born, and if you are never born, you won’t exist to come back and kill your grandfather, which means you will be born.

In the mid-1980s, physicist Igor Novikov used quantum mechanics to develop the “self-consistency theorem,” which demonstrates that there is no actual possibility of changing history with a time machine. The events of history cannot be altered, according to Novikov. Even if you went back in time and tried to kill your own grandfather, you wouldn’t be able to, because the events of history are fixed.

4) You don’t have nearly as much free will as you think you do.

Novikov’s “self consistency theorem” can be frustrating and difficult to accept. Don’t we all like to believe that we control our own destinies?

But what about the destiny of an inanimate object? Instead of your grandfather’s life, what if we were dealing with the movement of pool balls? Imagine a time machine set up so that a pool ball shot into the time machine came out a second earlier. Shouldn’t it be possible to aim a shot so that, when the pool ball emerged from the time machine a second in the past, it blocked the original shot that sent the pool ball into the machine in the first place?

Kip Thorne and his students examined the paradox of the “impossible pool shot” and came up with a compromise. They propose that, no matter how hard you tried to line up the shot perfectly, there would be some unplanned angle to the original shot. The ball would then come out of the machine at a slightly askew angle, thereby clipping the first ball just enough to send it into the time machine slightly askew, and the whole thing would continue in an endless perfect loop.

In other words, a physicist would argue that once you’ve seen your destiny by traveling to the future, there is nothing you can do to change it.

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What do you think of Goldberg’s four rules of time travel? Do you think time travel fiction should follow rules? Do you believe that writers of science fiction should keep the “science” in mind while plotting? Please share your thoughts in the comments!

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Julie Eshbaugh is represented by Natalie Fischer of the Bradford Literary Agency. You can read her blog here and find her on Twitter here.